1. Field of the Invention
The present invention relates to an ink set suitable for ink jet recording and an ink jet recording method, and more particularly to an ink set and an ink jet recording method which can realize images possessing excellent lightfastness and quality.
2. Background Art
Ink jet recording is a recording method wherein a recording liquid is ejected from fine nozzles provided on a recording head, for example, by vibration caused by a piezoelectric element or through the action of thermal energy to perform recording. The ink jet recording has advantages including quiet operation at the time of recording, recording of high-resolution images at a high speed through the use of a high-density head, and low running cost.
In the formation of color images by ink jet recording, at least three primary colors of yellow ink, magenta ink, and cyan ink, or four primary colors of yellow ink, magenta ink, cyan ink, and black ink are mainly used. Further, in recent years, a method has been proposed wherein a plurality of magenta inks different from each other in color density and a plurality of cyan inks different from each other in color density are used to yield higher definition images. The advance of such ink jet recording technology has realized the production of images having high quality comparable with photographs. This has lead to a tendency toward the use of output images in a wide variety of applications, and storage stability, such as lightfastness, of images is being regarded as important.
In order to improve the lightfastness of black images, Japanese Patent Laid-Open No. 44851/2000 proposes the incorporation of a dye or a pigment and a specific compound, and Japanese Patent Laid-Open No. 130557/1998 proposes the utilization of a specific black dye.
The present inventors have now found that the use of two black ink compositions, one of which contains a specific colorant, can realize images which possesses excellent lightfastness and, at the same time, have high quality comparable with photographs. The present invention has been made based on such finding.
Accordingly, it is an object of the present invention to provide an ink set which can realize images possessing excellent lightfastness and quality.
According to one aspect of the present invention, there is provided an ink set comprising at least a first black ink composition and a second black ink composition, wherein
said first black ink composition has a lightness (L*) in the range of 1 to 30 as calculated from the spectral transmittance of a 1000-fold dilution of said first black ink composition, and
said second black ink composition has an L* value in the range of 35 to 80 as calculated from the spectral transmittance of a 1000-fold dilution of said second black ink composition and contains a compound represented by formula (I) as a colorant: 
wherein
R represents a C1-6 alkyl group, a hydroxyl group, or a group xe2x80x94NR1R2 in which R1 and R2 each independently represent a hydrogen atom or a C1-6 alkyl group; and
M represents an alkali metal or a group xe2x80x94NR1R2 in which R1 and R2 each independently represent a hydrogen atom or a C1-6 alkyl group.
The ink set of the present invention can advantageously improve image quality, especially lightfastness and gradation, and can reduce graininess in a middle to high lightness region of images. Further, the ink compositions constituting the ink set according to the present invention, when used in combination with color inks, can yield good mixed images. In addition, the ink compositions constituting the ink set according to the present invention can satisfy various properties required of inks used in ink jet recording, for example, nozzle anti-clogging properties and storage stability.
The ink set according to the present invention comprises at least two ink compositions, that is, at least a first black ink composition and a second black ink composition.
The first black ink composition has a lightness (L*) in the range of 1 to 30 as calculated from the spectral transmittance of a 1000-fold dilution of the first black ink composition. On the other hand, the second black ink composition has an L* value in the range of 35 to 80, preferably 45 to 65, as calculated from the spectral transmittance of a 1000-fold dilution of the second black ink composition. In the present invention, an ink composition is diluted with water.
Further, according to the present invention, the second black ink composition contains the compound represented by formula (I) as a colorant. In formula (I), R represents a C1-6 alkyl group (preferably a methyl group), a hydroxyl group, or a group xe2x80x94NR1R2 in which R1 and R2 each independently represent a hydrogen atom or a C1-6 alkyl group and preferably R1 and R2 each represent a hydrogen atom. M represents an alkali metal (preferably sodium, lithium, or potassium) or a group xe2x80x94NR1R2 in which R1 and R2 each independently represent a hydrogen atom or a C1-6 alkyl group. Specific examples of the compounds represented by formula (I) include compounds having the following structures represented by formulae (II) to (IV). 
Specific examples of preferred compounds represented by formula (I) include water-soluble C.I. Sulfur Black 1. This dye is commercially available from Clariant under the designation Duasyn Black SU-SF liq VP 2287.
The amount of the compound represented by formula (I) added may be properly determined. However, the amount of this compound added is preferably 1 to 5% by weight based on the ink composition.
Any of dyes and pigments may be used as the colorant in the first black ink composition.
Dyes usable herein include those used as colorants in ink compositions for ink jet recording, for example, dyes which fall into categories of acidic dyes, direct dyes, basic dyes, reactive dyes, sulfur dyes, and food dyes according to the color index. Examples thereof include: C.I. Direct Black 19, C.I. Direct Black 22, C.I. Direct Black 51, C.I. Direct Black 56, C.I. Direct Black 71, C.I. Direct Black 74, C.I. Direct Black 75, C.I. Direct Black 77, C.I. Direct Black 154, C.I. Direct Black 168, and C.I. Direct Black 171; C.I. Acid Black 2, C.I. Acid Black 7, C.I. Acid Black 24, C.I. Acid Black 26, C.I. Acid Black 31, C.I. Acid Black 52, C.I. Acid Black 63, and C.I. Acid Black 112; and C.I. Reactive Black 3, C.I. Reactive Black 4, C.I. Reactive Black 7, C.I. Reactive Black 11, C.I. Reactive Black 12, C.I. Reactive Black 17 and C.I. Reactive Black 31. They may be used alone or as a mixture of two or more. Although the amount of these dyes added may be properly determined, the amount of these dyes added is preferably in the range of 4 to 7% by weight.
Any of inorganic pigments and organic pigments may be used as the pigment. The inorganic pigment may be carbon black produced by a conventional method. Organic pigments include azo pigments, phthalocyanine pigments, and anthraquinone pigments. These pigments may be used alone or as a mixture of two or more. These pigments may also be used as a mixture with dyes. The amount of these pigments added may be properly determined. The amount of these pigments added, however, is preferably in the range of 2.5 to 7% by weight.
The solvent in the first black ink composition and the solvent in the second black ink composition each preferably comprises water and a water-soluble organic solvent.
Water is preferably deionized water. The content of water may be properly determined depending upon the type and amount of organic solvents described below and the properties of desired ink compositions. In general, however, the water content is preferably in the range of 10 to 70% by weight based on the ink composition.
The water-soluble organic solvent is preferably a low-volatile solvent from the viewpoint of preventing clogging. Examples of preferred low-volatile solvents include: polyhydric alcohols, such as ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, 1,3-propanediol, 1,5-pentanediol, 1,2-hexanediol, 1,2,6-hexanetriol, and glycerin; glycol ethers such as ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monobutyl ether, triethylene glycol monomethyl ether, triethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, and dipropylene glycol monomethyl ether; nitrogen-containing solvents such as formamide, dimethylformamide, diethanolamine, triethanolamine, 1,3-dimethyl-2-imidazolidinone, 2-pyrrolidone, and N-methyl-2-pyrrolidone; and sulfur-containing solvents such as thiodiglycol and dimethyl sulfoxide. These water-soluble organic solvents may be used alone or as a mixture of two or more. The amount of the water-soluble organic solvent added may be properly determined. The amount of the water-soluble organic solvent added, however, is preferably about 3 to 40% by weight from the viewpoint of clogging and print quality.
Further, according to a preferred embodiment of the present invention, the first black ink composition and the second black ink composition contain an acetylene glycol nonionic surfactant or a lower alkyl ether of a glycol compound. The incorporation of both the acetylene glycol nonionic surfactant and the lower alkyl ether of a glycol compound is more preferred from the viewpoint of realizing feathering-free images.
The acetylene glycol nonionic surfactant may be commercially available one, and examples thereof include Olfine E 1010, Olfine STG, and Surfynol 104 E (all the above products being available from Nissin Chemical Industry Co., Ltd.). The amount of the acetylene glycol nonionic surfactant added is preferably in the range of 0.01 to 3% by weight based on the total weight of the ink.
Examples of preferred lower alkyl ethers of glycol compounds include diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, propylene glycol monobutyl ether, and dipropylene glycol monobutyl ether. Although the amount of the lower alkyl ether of a glycol compound added may be properly determined, the amount of this lower alkyl ether added is preferably about 3 to 40% by weight, more preferably 3 to 30% by weight.
According to a preferred embodiment of the present invention, the first black ink composition and the second black ink composition contain various optional additives which impart desired properties to the inks. Specific examples of optional additives usable herein include viscosity modifiers, such as sodium alginate and polyvinyl alcohol, hydroxides of alkali metals, such as potassium hydroxide, sodium hydroxide, and lithium hydroxide, pH adjustors, such as ammonium phosphate, preservatives (for example, Proxel XL-2 manufactured by Avecia), rust preventives, antioxidants, antimolds, and chelating agents.
The ink set according to the present invention may be used in monochromic printing, or may be used in combination with yellow ink, magenta ink, and cyan ink to constitute a color ink set which is used in color printing. Alternatively, a method may be adopted wherein two magenta ink compositions different from each other in color density and two cyan ink compositions different from each other in color density are provided, are combined with yellow ink, and are further combined with the ink set according to the present invention. The ink set according to the present invention, which has been combined with color inks other than yellow ink, magenta ink, and cyan ink, falls within the scope of the present invention.
The ink set according to the present invention may be used in recording methods using an ink composition. Recording methods using an ink composition include, for example, an ink jet recording method, a recording method using writing utensils, such as pens, and other various printing methods. In particular, the ink composition according to the present invention is preferably used in an ink jet recording method wherein droplets of an ink composition are ejected and deposited onto a recording medium. The ink jet recording method may be any of a method wherein the vibration of a piezoelectric element is utilized to eject ink droplets, a method wherein thermal energy is utilized, and other type of ink jet recording methods.